Method and apparatus for optimizing spectrum utilization by a cognitive radio network

ABSTRACT

A technique for a secondary communication system to utilize spectrum designated to another (or primary) communication system is provided. By ranking a plurality of secondary base stations based on base station transmit power, calculated required transmit power and path loss, a set of criteria is developed for selecting a highest ranked secondary base station for operation within a primary&#39;s spectrum. The ranking may be adapted based on mobility of the secondary&#39;s subscriber; and as such the secondary system communicates within the primary&#39;s spectrum using the adaptively ranked base stations. Channel selection may also be ranked. The technique and apparatus allows a cognitive radio (CR) network to operate within an incumbent network&#39;s spectrum.

FIELD OF THE INVENTION

The invention generally relates to communication systems and moreparticularly to cognitive radio networks and the utilization ofspectrum.

BACKGROUND OF THE INVENTION

Wireless products and services have continued to expand to the pointthat finite resources of available communication spectrum are beingoverwhelmed. Industry has been forced to make dramatic changes, as itmust adapt to accommodate the exponential demand on spectrum access,efficiency and reliability.

The Federal Communications Commission (FCC) in the United States, andits counterparts around the world, allocate radio spectrum acrossfrequency channels of varying bandwidth. Various bands may cover, forexample, broadcast radio, television, cellular phones, citizen's-bandradio, pagers and so on. As more devices go wireless, an increasinglycrowded radio spectrum needs to be shared. Although the radio spectrumis almost entirely occupied, based on current methods of fixed spectrumallocation and licensing, not all devices use portions of the licensedradio spectrum at the same time or location. At certain times, a largepercentage of the allocated spectrum may be sitting idle, even though itis officially assigned and accounted for.

Cognitive radio is a paradigm for wireless communication in which eithera network or wireless device alters its transmission or receptionparameters to avoid interference with licensed or unlicensed incumbentusers. Cognitive radios must implement methods to avoid selecting anoccupied frequency, so as to avoid interference to the incumbent device.Cognitive radio systems utilizing several base stations presentadditional challenges to spectrum sharing in terms of coordination andmanagement that avoids interference amongst both the cognitive networkitself and the incumbent system.

Accordingly, it would be highly desirable to optimize spectrum sharingamongst cognitive radio systems and incumbent systems, particularly inthe case of multiple cognitive base stations.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying figures where like reference numerals refer toidentical or functionally similar elements throughout the separate viewsand which together with the detailed description below are incorporatedin and form part of the specification, serve to further illustratevarious embodiments and to explain various principles and advantages allin accordance with the present invention.

FIG. 1 is a system diagram of a cognitive radio network operating withinan incumbent network's spectrum in accordance with an embodiment of theinvention.

FIG. 2 a flowchart of secondary base station selection in accordancewith an embodiment of the invention.

FIG. 3 is a block diagram of a cognitive mobile subscriber in accordancewith an embodiment of the invention.

FIGS. 4A and 4B illustrate a flowchart of base selection in accordancewith an embodiment of the invention.

FIG. 5 is a flowchart of adaptive base selection based on mobility inaccordance with an embodiment of the invention.

Skilled artisans will appreciate that elements in the figures areillustrated for simplicity and clarity and have not necessarily beendrawn to scale. For example, the dimensions of some of the elements inthe figures may be exaggerated relative to other elements to help toimprove understanding of embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Before describing in detail embodiments that are in accordance with thepresent invention, it should be observed that the embodiments resideprimarily in combinations of method steps and apparatus componentsrelated to optimum base station or an operational channel selection by asubscriber in a cognitive radio (CR) network, where the cognitivenetwork is seeking to utilize an incumbent network's spectrum. Themethod may be utilized for optimum operational channel selection in thecase of one or more base stations. The cognitive radio system of thepresent invention may employ one or more CR base stations, eachoperating over one or more channels in an incumbent network's spectrum.Thus, selection of an optimum base station and/or optimum channel foroperation without interfering with incumbent systems is highlybeneficial to both systems. The method of selecting the optimum basestation and/or optimum channel may further encompasses adapting theselection based on mobility of a CR subscriber. The CR subscriber ranksCR bases stations based on certain parameters, selects an optimum basestation for operation, and then continuously re-ranks the base stationsbased on mobility of the CR subscriber. Reliable communications can thusbe maintained by the adaptive ranking and selection of the CR basestation.

Accordingly, the apparatus components and method steps have beenrepresented where appropriate by conventional symbols in the drawings,showing only those specific details that are pertinent to understandingthe embodiments of the present invention so as not to obscure thedisclosure with details that will be readily apparent to those ofordinary skill in the art having the benefit of the description herein.

In this document, relational terms such as first and second, top andbottom, and the like may be used solely to distinguish one entity oraction from another entity or action without necessarily requiring orimplying any actual such relationship or order between such entities oractions. The terms “comprises,” “comprising,” or any other variationthereof, are intended to cover a non-exclusive inclusion, such that aprocess, method, article, or apparatus that comprises a list of elementsdoes not include only those elements but may include other elements notexpressly listed or inherent to such process, method, article, orapparatus. An element proceeded by “comprises . . . a” does not, withoutmore constraints, preclude the existence of additional identicalelements in the process, method, article, or apparatus that comprisesthe element.

In the description herein, numerous specific examples are given toprovide a thorough understanding of various embodiments of theinvention. The examples are included for illustrative purpose only andare not intended to be exhaustive or to limit the invention in any way.It should be noted that various equivalent modifications are possiblewithin the spirit and scope of the present invention. One skilled in therelevant art will recognize, however, that an embodiment of theinvention can be practiced with or without the apparatuses, systems,assemblies, methods, components mentioned in the description.

For the purposes of this application, a primary system is an incumbent(or pre-existing) user of a frequency spectrum. Incumbents are oftenlicensed users, such as TV broadcasters in the TV spectrum. The primarysystem may contain a television receiver that is within a servicecontour, licensed wireless microphones, or other systems governed by theFCC or other regulatory body. A secondary system may comprise a varietyof communication networks, for example a public safety network, cellularnetwork or other system having at least one master node, for example abase station, and slave node, typically in the form of a subscriberunit, such as a portable radio, cell-phone, PDA or the like. Secondarysystems may also include ad hoc networks, where multiple unitscommunicate among each other, and may dynamically designate master andslave nodes. There is an increased interest in deploying public safetydevices in secondary spectrum to improve interoperability, range anddata throughput and maintain mission criticality in case of emergenciesand disaster management scenarios.

FIG. 1 shows a system diagram of an incumbent device 102 and a pluralityof secondary base stations 104 (labeled CR BS 1, CR BS 2, CR BS 3)seeking to share spectrum within the incumbent's spectrum in thegeographic area 106 in accordance with an embodiment of the invention.Each incumbent transmitter 102 typically updates a policy andgeo-location database 108 with any changes in its transmission andreception parameters, protected contours etc. Database 108 alsoincorporates all co-channel and adjacent channel incumbent interferenceprotection requirements. The plurality of secondary base stations 104operate in accordance with the data provided by database 108. Secondarybase stations 104 may periodically sense for the presence of anincumbent and vacate that frequency or spectrum as soon as any incumbentsystem activity is detected. In this embodiment, relative to theincumbent transmitter 102, CR BS 1 is operating on an adjacent channel,CR BS 2 is operating on a co-channel, and CR BS 3 is operating onanother adjacent channel. In accordance this embodiment, a CR mobilesubscriber, CR MS 120, selects one of the secondary base stations CR BS1, CR BS 2, or CR BS 3 based on ranking criteria which is determined asprovided herein.

In accordance with this embodiment, ranking criteria for secondary basestation selection by the CR mobile subscriber 120 is based on: maximumallowed CR mobile transmit power versus location (as may be computed bythe base station from the database 108 and broadcast by the base stationon the control channel, or may be computed locally based onsensing/scanning); required CR mobile transmit power estimated from CRbase station signal measurements; frequency separation of the CR baseand mobile station from active incumbents; and link budget estimated bythe CR mobile for prospective CR base stations. Estimated link budget,maximum allowed transmit power levels, and potentially local noiselevels per channel are all taken into account to achieve optimum basesite or channel selection (also referred to as Quality of Service orQoS) for the CR mobile subscriber 120. The local noise level per channelof both the base station and the CR mobile are taken into account forranking purposes. The base station can broadcast the local noise levelfor each channel along with the list of candidate channels. The database108 may be stored within the CR mobile subscribers for the locationswithin which it operates. Database 108 is updated with current policyand geo-location information by incumbent 102. Each base station isranked based on estimated link budgets (which require knowledge of pathloss) and channel usability (which is time and location varying). Thusactual link budgets, mobility and transmit power levels are all takeninto account to achieve optimum QoS for the CR mobile subscriber 120.

The selection of an optimum communications channel by each base stationcomprises obtaining a list of candidate channels from a geo-locationdatabase based on location of the base station; determining a maximumallowed transmit power level on each of a plurality of candidatechannels based on geo-location; measuring a power level and optionally anoise/interference level on each of the plurality of candidate channels;computing an estimated link budget for each candidate channel, whereinthe estimated link budget comprises a difference between the maximumallowed transmit power level and the corresponding measured power levelfor the candidate channel; and selecting the channel that has thehighest estimated link budget for communications.

The maximum allowed CR mobile transmit power vs. location parameter iscalculated via the geo-location database 108 or through spectral sensingmeasurements made by the CR mobile subscriber 120, based on allco-channel and adjacent channel transmission requirements associatedwith the incumbent(s). The method essentially selects a channel thatsupports the highest usable dynamic range (or signal to noise plusinterference ratio) for communications. The maximum allowed transmitpower level for each channel may be determined through geo-locationmeans, through sensing means, or a combination of geo-location andsensing means. Geo-location based power determination methods indicate amaximum CR unit power level based on the unit's proximity (in distanceand/or operating frequency) to other users (e.g., incumbents) of thespectrum. Spectral sensing based power determination methods indicate amaximum CR unit power level based on sensing measurements of other users(e.g., co- and adjacent channel incumbents) of the spectrum. In bothcases, maximum CR unit transmit power levels are limited by proximity toother users and their interference protection requirements.

The maximum allowed CR transmit power vs. location or environment isgenerally determined to be a safe maximum power level limit for the CRdevice to utilize, which ensures that harmful interference does notoccur to incumbent users. Harmful interference levels are generallydetermined to be interference or undesired signal levels that exceedprescribed interference protection ratios, such as desired-to-undesired(D/U) ratios. These ratios are often determined based on incumbent type(e.g., DTV service, analog TV service, etc.).

Based on the maximum allowed transmit power level and measuredparameters, the CR unit (i.e., CR base station or CR subscriber) mayestimate both the downlink and uplink channel, using either active orpassive channel measurements. Active channel measurement involves oneunit sending out channel sounding bauds or other signals, to allowanother unit to measure the received signal to estimate the channel(e.g., to determine path loss, etc.). Passive channel measurementstypically involve measuring background noise and interference levels oneach channel (see below). Active channel measurements are discussed inconjunction with Table I while passive channel measurements arediscussed in conjunction with Table II.

In Time Division Duplex (TDD) communication systems, the downlinkchannel is often estimated to be the same as the uplink channel(especially over short time frames). Thus, the path loss on the downlinkchannel can be estimated to be the same as the uplink channel. InFrequency Division Duplex (FDD) systems, the uplink and downlinkchannels often have to be estimated separately. Typically multiplechannels (e.g., channel N−1, N, and N+1) are available for CR use, eachwith differing maximum allowed transmit power levels. An example ofranking criteria by a CR subscriber for a TDD system is shown in Table Ibelow:

TABLE I Secondary Secondary BS 1 Channel BS 2 Secondary BS 3 RankingCriteria (N − 1) Channel (N) Channel (N + 1) Maximum allowed 30 dBm 10dBm 40 dBm TX Power of BS Measured RSSI at −70 dBm −75 dBm −90 dBm MS(from BS) Estimated Path Loss 100 dB 85 dB 130 dB Required TX Power 20dBm 5 dBm 50 dBm (>max) of MS Rank 2 1 3

The required transmit power of the CR mobile subscriber from Table Iabove can be calculated in the manner shown below. This method assumesactive sounding of the channel (e.g., the BS sends out a signal on eachcandidate channel to allow MSs to estimate the path loss). This signalmay be sent during normal communications (e.g., preambles, sync bauds,etc.) or it may serve the specific purpose of sounding the channel(e.g., sounding bauds). Typically, existing secondary base stations in asystem will already be actively transmitting to other users in a system,so no special additional signaling will be required.

The received RSSI for the CR MS is given by,

RSSI_(MS)=EIRP_(BS)−PL=P _(TX,BS) +G _(TXA,BS) +G _(RXA,MS)−PL

where P_(TX,BS) is the base transmit amplifier power output (TPO) levelof the base station, and G_(TXA,BS) is the transmit antenna gain of thebase station, (P_(TX,BS)+G_(TXA,BS)) is the Effective Isotropic RadiatedPower (EIRP) level of the BS signal, PL is downlink path loss andG_(RXA,MS) is receive antenna gain of MS. All components of the equationare typically represented logarithmically (in dB format). Note that themeasured received RSSI level may have to be adjusted to remove (i.e.,subtract out) the effects of known incumbent transmitters on co- and/oradjacent channels, since these terms represent interference, and not thedesired signal.

Thus, the downlink (base to mobile) path loss is

PL=(P _(TX,BS) +G _(TXA,BS))+G _(RXA,MS)−RSSI_(MS)

The MS can estimate path loss based on the knowledge of BS EIRP leveland its own antenna gains. The base would typically broadcast thetransmit power levels and antenna gain (or EIRP) that it is utilizing oneach channel to allow MSs to readily estimate the path loss (as above)for each channel. Alternatively, the MS could compute the maximum BSEIRP level based on the BS location, which can be broadcast by the BS.Note that filtering may take place on measured RSSI values to reduce thevariance of this measure. Each CR BS may also broadcast its localnoise-plus-interference floor level for each channel so that the CR MScan make a more informed ranking of the uplink budgets. By knowing itsown local noise-plus-interference floor, a MS can make a more informedranking of the downlink budget for each BS as well.

For Time Division Duplex systems, the path loss (PL) can be assumed tobe reciprocal (that is, the path loss in the downlink is equal to thepath loss in the uplink, as long as the antenna gains remain constant).If the channel is not reciprocal (as in frequency division duplexsystems), the path loss should be estimated on each channel (e.g.,uplink and downlink), and the appropriate path loss estimate for thechosen frequency should be utilized.

The maximum RSSI based on allowed MS Tx EIRP and PL is then given by,

RSSI_(TGT,BS) =P _(TX,MS) +G _(TXA,MS) +G _(RXA,BS)−PL,

where P_(TX,MS) is the required transmit power output (TPO) of the MS toachieve the target RSSI level at the BS (RSSI_(TGT,BS)). Thus, if allother factors are equal, such as noise levels, the MS would select thechannel with the minimum path loss to transmit on, which tends tomaximize received signal-to-noise ratio (SNR). In CR systems though, thenoise floor and maximum allowed transmit power on each channel typicallyvaries with device environment (e.g., device location or sensedenvironment), which adds variability into the above equation. Themaximum allowed transmit power level cannot be exceeded on any givenchannel, and may limit the achievable target RSSI level at the basestation. (This situation is shown in the last column of Table I.)

Substituting in the estimated downlink path loss (for reciprocalchannels, where the uplink path loss equals downlink path loss, asdescribed above):

RSSI_(TGT,BS) =P _(TX,MS) +G _(TXA,MS) +G _(RXA,BS) −P _(TX,BS) −G_(TXA,BS) −G _(RXA,MS)+RSSI_(MS)

yields:

P _(TX,MS)=RSSI_(TGT,BS) −G _(TXA,MS) −G _(RXA,BS) +P _(TX,BS) +G_(TXA,BS) +G _(RXA,MS)−RSSI_(MS)

Hence, knowing the target received signal strength (RSSI_(TGT,BS)) forthe secondary base station (e.g., based on a target bit error rate forthe desired modulation), the antenna gains and the transmit power of thesecondary base station (P_(TX,BS)) allows the required transmit power ofthe CR mobile subscriber (P_(TX,MS)) to be calculated as shown above.Note that the transmit and receive antenna gains may cancel out of theabove equation if they are equal, the transmit antenna gain is oftenequal to the receive antenna gain. The selected channel would thentypically be the channel with the minimum path loss, or equivalently,the lowest required MS transmit power level (as long as that power leveldoes not exceed the maximum allowed power level for that particularchannel).

Also note that the target RSSI level at the base station may be directlyimpacted by the local noise (plus interference) level at the basestation, as follows:

RSSI_(TGT,BS) =C/I _(TGT,BS)+(N+I)_(BS)

where C/I_(TGT,BS) is the carrier to interference ratio target at the BS(based on desired bit error or frame error rates). The noise plusinterference measure is typically measured by the base station (duringquiet periods) for each channel that it is utilizing, and the values maybe broadcast over a control channel. The MS units may then take intoaccount local (BS) receiver noise levels in their computations, whichwill in turn influence the required MS transmit power levels andchannel/base site selection.

At the CR MS 120, referring to the Table I above using the describedactive channel sounding method, even though the signal from CR BS 2 isweaker (by 5 dB) at the CR MS 120 (based on RSSI), the CR MS 120 selectsCR BS 2 since the path loss estimate is better for CR BS 2. Note that CRBS 2 is actually transmitting with the least transmit power levelthough, due to its geo-location (e.g., proximity to a co-channelcontour).

Alternatively, in another embodiment of the invention, passiveestimation of the channel can be performed. Passive estimation does notrequire active sounding of the channel as described above. Signal tointerference plus noise ratio (SINR) can be blindly estimated on eachcandidate channel and used for channel selection and ranking purposes.This estimation can be performed independently at the BS or MS, and doesnot require an active signal to be sent out on the channel (e.g.,sounding bauds to estimate path loss). The estimated SINR can becalculated for each channel by dividing the maximum (BS or MS) allowabletransmit power level P_(TX) (for a given channel (ch), as determined bythe unit or system's environment or location) by the measured noise plusinterference level for that channel. Received SINR ratios are the ratioof the transmit power minus the path loss divided by the interferenceplus (uncorrelated) noise power components observed at the unit for thatoperating channel:

SINR_(ch)=[EIRP_(ch)−PL]/[P _(N+Ich)]

which is in turn representative of the achievable modulation level oneach link when communicating in a properly scheduled or loaded network.It is assumed that the Effective Isotropic Radiated Power (EIRP) figureabove takes into account transmit antenna gains. The path loss (PL)values can be assumed to be equal in the described blind estimationmethod (and thus do not affect relative ranking results betweenchannels). As above, the channel with the highest estimated SINR istypically utilized for communication. The noise plus interference levelsare typically measured during a quiet period in the system.

Stated another way, the channel (or BS operating frequency) with thehighest estimated passive link budget can be chosen for communication.Stated logarithmically in dB (as above), the passive link budget(LB_(est,ch)) can be estimated for each channel as:

LB_(est,ch)=EIRP_(ch) −P _(N+I,ch)

where P_(N+I,ch) is the measured background noise and interference levelon the channel (typically averaged at the receiver during a quietperiods on the channel). Typically, EIRP_(ch) is taken to be the maximumallowed EIRP per channel, as determined by a geo-location database orthrough sensing means. These levels are typically dictated by operatingregulations in the band. EIRP_(ch) may alternatively represent the powerlevel that is chosen to be transmitted on each channel, if it is furtherconstrained due to other reasons (e.g., power amplifier limitations).

Table II illustrates the passive channel estimation process describedabove:

TABLE II Secondary Secondary BS 1 BS 2 Secondary BS 3 Ranking CriteriaChannel (N − 1) Channel (N) Channel (N + 1) Maximum allowed 25 dBm 10dBm 36 dBm TX Power of BS Measured P_(N+I,ch) −70 dBm −90 dBm −60 dBm(typ. at BS) Estimated Link 95 dB 100 dB 96 dB Budget (LB_(est,ch)) Rank3 1 2

Thus in the case of passive channel estimation, the CR unit (e.g., BS)would choose to transmit on channel N, even though that channel allowsthe lowest maximum transmit power (or EIRP) level. The estimated linkbudget on that channel is the best available though, due to the lowernoise plus interference levels on that channel. The CR BS may utilizethis type of passive channel selection method at power up, before a linkwith a CR MS is established. Accordingly, examples have been provided ofboth active (Table I) and passive (Table II) channel measurements.

Moving on to FIG. 2, there is shown a flowchart of a method forsecondary base station selection and mobility in accordance with anembodiment of the invention. The system illustrated is a TDDcommunications network.

Beginning at 202, the CR base stations 104 obtain a list of candidatechannels along with maximum allowed transmit power (e.g., fromgeo-location database 108) and then proceed to sense all incumbentchannels at 204. At 206, the secondary base stations 104 determineavailable TV Whitespace frequencies and selects, at 208, the bestoperating channel for its location (typically through the passivechannel estimation methods described above). Each CR base station 104typically selects the best channel to operate on using a combination ofthe geo-location information from the database 108 and passive channelestimation results. Each base station starts operating the CR network onthe best available TV channel and broadcasting CR network data at 210.In the embodiment of FIG. 1, this would equate to CR BS1 operating andbroadcasting on f1 (channel N−1), CR BS2 on f2 (channel N) and CR BS3 onf3 (channel N+1).

The CR MS 120 scans for all available CR channels, generated from the CRstations at 212. In the embodiment of FIG. 1, CR MS 120 is thus scanningthe spectrum for a CR channel from each CR base station 104 in order toselect one of the CR base stations for operation. The CR MS 120 may scanall frequencies for incumbents, at 212. For active channel estimation,the CR MS 120 measures the CR base station signal strength on allchannels at step 214, as described above. For the embodiment of FIG. 1this would equate to measuring received BS signals strength measurements(RSSI) on each available channel. Transmission and reception parametersare then estimated at step 216 for all available CR base stations basedon knowledge of each BS's transmit power level, and possibly its localnoise floor level, as described above. Recall that each BS isconstrained to transmit with a maximum allowed power level, typicallydictated by its operating location (and its proximity to incumbentsystems). Determining a MS transmit channel, as discussed above, isbased on path loss between each of the CR base station BS1, BS2, BS3 andthe CR mobile subscriber, with the path loss being determined by thedifference between the transmit power of the CR base station and thereceived signal strength of the mobile subscriber.

A base station selection and channel selection algorithm is run at 218.Further details pertaining to the base station selection and channelselection algorithm will be described in conjunction with FIG. 4.Briefly the base station selection and channel selection algorithm ranksthe base stations based on the estimated path loss values, requiredtransmit power of the mobile subscriber (to reach the base) and transmitpower of the base and noise-plus-interference levels at each end of thelink. This process may include utilizing the noise plus interferencelevels at each end of the link, as described above. So, for example, atable such as Table I shown above might be generated for the embodimentof FIG. 1, depending on whether active or passive measurements aretaken.

The CR mobile subscriber 120 then joins the highest ranked secondarybase station at step 220. In the embodiment of FIG. 1 with the abovetable this would mean that MS 120 joins CR BS 2. The CR mobile thusjoins the network at 222 operating on, but not interfering with, theincumbent's spectrum.

FIG. 3 is a block diagram of a cognitive mobile subscriber (CR MS) 120in accordance with an embodiment of the invention. In this embodiment alocal geo-location database 302 is shown integrated within the CR mobilesubscriber 120. The geo-location information provided by the database302 includes information as to the type of incumbents that the CRsubscriber should look for along with contour information for thatincumbent and the maximum allowable transmit power for a CR unit at aparticular time and location along with other relevant transmission andreception policies and parameters.

The CR mobile subscriber 120 may further include a software definedradio section 304 having a plurality of air interfaces 306 with which tomeasure signal quality of known secondary base stations. So, forexample, CR subscriber 120 would measure the signal quality of CR basestations 104 in FIG. 1. A spectrum sensor 308 may be used to measure theprimary systems in the vicinity, either to help validate database 108information, or to determine maximum transmit power levels based onsensing measurements as described above. The spectrum sensor 308provides primary base station signal quality (SQ) measurements tocognitive secondary mobile subscriber. The secondary mobile subscriber120 includes cognitive engine 310 which performs selection of the basestation based on the sensed results and then adapts these results inconjunction with mobility of the CR MS 120. Thus the secondary mobilesubscriber selects via cognitive engine 310, a base station to join thathas sufficient link budget at that particular point in time and at thatlocation.

FIGS. 4A/4B illustrate a flowchart of the adaptive CR base stationranking algorithm 400, such as performed by the cognitive engine 310 ofFIG. 3, in accordance with an embodiment of the invention. Dashed linesteps are optional and may be performed as part of other steps asspecified in some cases below. The secondary mobile subscriber startsthe routine at step 402 by obtaining a list of candidate channels fromlocal geo-location database or by sensing the entire spectrum ofoperation if geo-location data is not available locally. The CR mobiledetermines the incumbent data and maximum allowed transmit power at 404.The incumbent data and maximum allowed transmit power can be determinedfrom the geo-location database, by sensing, or by combination ofgeo-location database and sensing algorithms at 404. The CR mobilemeasures secondary base station signal strength for each channel at 406.The secondary subscriber goes to the first channel at 408 and mayoptionally check to see if a primary (incumbent) device is present onthat channel at 410. If a primary is present at 410 then the CRsubscriber marks the channel as not available at 432. If more channelsare available at 420 the routine returns to step 410. If no morechannels are available, then the channels are ranked and listed at 422based on channel usability (comprising maximum allowed transmit power,link budget), the required power for channel access, and channelseparation from active incumbent. The highest ranked BS and its channelare selected by CR mobile for operation at 434.

If no primary was present at 410, the routine checks to see if asecondary system is present at 424. The determination of whether aprimary is present or not can be made based on geo-location database orspectrum sensing or based on algorithms utilizing both geo-locationdatabase and spectrum sensing. If a secondary is present without aprimary 410, 424, then a calculation of minimum power required to reachthe secondary base station is taken at 426. An optional check is made at428 to determine if the primary base station is active on an adjacentchannel. Both co-channel and adjacent channel constraints can be takeninto account while determining the maximum allowed transmit power levelper channel in step 404. The presence of the incumbents on co- oradjacent channels will have the impact of modifying the maximum allowedCR transmit power level, and thus the ranking at 418—which would bereflected in the allowed EIRP level. A check is made at 420 for morechannels. If no more channels need to be analyzed then the ranking iscompleted at 422 and the highest ranked base station is selected at 434.Again, additional channels, if any are analyzed and once completed at420 the channels are ranked at 422 and a selection is made at 434.

FIG. 5 is a flowchart for maintaining operation in the presence ofmobility in accordance with an embodiment of the invention. At thispoint of operation, the highest ranked base station has been selectedwith its channel of operation using the technique as provided in theprevious embodiment. The mobility of the secondary subscriber however,impacts whether the currently selected base station and channel willcontinue to be used or whether a switch to another base station and/orchannel needs to be made. Algorithm 400 continues to run in order tomaintain a list of available secondary channels. At 504, the CRsubscriber continuously performs in-band and out-of-band sensing whileoperating on the channel selected for operation from the algorithm 400.During the in-band sensing, RSSI values (Pr1, Pr2, Pr3) are sensed at506 where Pr1 and Pr3 are the lower and upper adjacent channel,respectively, where channels Pr1 and Pr3 are affected by CRtransmissions on channel Pr2. If an incumbent is detected at 508, thenthe highest ranked candidate channel is selected for operation at 524.If no incumbent is detected at 508, then the required power of thesecondary subscriber to reach the secondary base is compared at 518 tothe maximum allowed secondary base power and, if greater, then thehighest ranked candidate channel is selected for operation at 524. Ifthe required power to reach the base is less than the secondary base'smaximum power (thus still being reachable) then the selected channelremains unchanged and in-band sensing continues at 506.

The operating channel (Pr2) is concurrently sensed at 512 for signalquality (SQ). If the signal quality is determined to be low over time at514, then the highest ranked candidate channel is selected for operationat 524.

Thus, the mobility is detected based on the presence of an incumbent 508or the power of the subscriber dropping below that needed to reach thesecondary base 518 or a drop in signal quality on the current operatingchannel 514. The measured incumbent signal level and CR base stationsignal quality may be integrated over a period of time to take intoaccount short term variations in measurements. If the CR mobile detectedthe presence of an incumbent or a drop in CR subscriber power or a dropin signal quality then it selects the highest ranked base station foroperation at 524 from the list at 502. Hence, the base station selectionoperating in accordance with the embodiments of the invention is able toremain adaptive within a mobile environment.

Accordingly, there has been provided a means for optimizing spectrumutilization amongst secondary systems (such as cognitive radio systems)and primary systems (such as incumbent systems). Mobile subscribersutilizing a plurality of base stations can now readily rank the basestations and/or operating channel and adapt the system to operating on adifferent base station and/or channel in response to mobility and otherchanges in the environment.

While systems exist that detect and mitigate interference using powercontrol and iterative transmit power updates, none of these existingsystems have ranked base stations and provided for the selection of anoptimum base station based on location and environmental conditions inorder to avoid interference to incumbents while operating in theirspectrum. Nor have these past systems adapted the rankings based onlocation and/or sensing while operating in a spectrum on secondary basiswhere the incumbents limit the maximum allowed transmit power and theoperating region of the base stations and the mobiles. While operatingin primary's spectrum, the mobile subscriber operating in accordancewith the embodiments of the invention adapts operating parameters andselects secondary base stations for operation not only based on thesignal quality of the base stations, but also based on the interferencegenerated to the incumbents due to its operation.

Those skilled in the art will appreciate that the above recognizedadvantages and other advantages described herein are merely exemplaryand are not meant to be a complete rendering of all of the advantages ofthe various embodiments of the present invention.

In the foregoing specification, specific embodiments of the presentinvention have been described. However, one of ordinary skill in the artappreciates that various modifications and changes can be made withoutdeparting from the scope of the present invention as set forth in theclaims below. Accordingly, the specification and figures are to beregarded in an illustrative rather than a restrictive sense, and allsuch modifications are intended to be included within the scope of thepresent invention. The benefits, advantages, solutions to problems, andany element(s) that may cause any benefit, advantage, or solution tooccur or become more pronounced are not to be construed as a critical,required, or essential features or elements of any or all the claims.The present invention is defined solely by the appended claims includingany amendments made during the pendency of this application and allequivalents of those claims as issued.

1. A method for selecting a communications channel in a CR system,comprising the steps of: determining a maximum allowed transmit powerlevel on each of a plurality of candidate channels; measuring a powerlevel on each of the plurality of candidate channels; computing anestimated link budget for each candidate channel, wherein the estimatedlink budget comprises a difference between the maximum allowed transmitpower level and the corresponding measured power level for the candidatechannel; and selecting the channel that has the highest estimated linkbudget for communications.
 2. The method of claim 1, wherein the step ofdetermining a maximum allowed transmit power level is determined usinggeo-location information.
 3. The method of claim 1, wherein the step ofdetermining a maximum allowed transmit power level is determined usingsensing information.
 4. A method for selecting a cognitive radio (CR)channel, comprising the steps at a CR mobile subscriber: scanningfrequency bands to determine the availability of one or more CR basestations at the location of the CR mobile subscriber; determining atransmit power level for each available CR base station; measuring areceived power level for each available CR base station; computing anestimated link budget for each candidate channel, wherein the estimatedlink budget comprises a difference between the transmit power level ofthe CR base station and the corresponding measured received power levelfor the CR base station; calculating required transmit power by the CRmobile subscriber to reach each CR base station and avoid interferenceto identified incumbents; ranking the CR base stations based on theestimated link budget, required transmit power and maximum allowedtransmit power level of the CR mobile subscriber; and selecting andutilizing a highest ranked CR base station and channel for transmissionfrom the CR mobile subscriber.
 5. The method of claim 4, wherein thestep of calculating required transmit power by the CR mobile subscribercomprises determining the transmit power level required to achieve atarget received signal strength indicator (RSSI) level at the CR basestation.
 6. The method of claim 4, wherein the step of calculatingrequired transmit power by the CR mobile subscriber further comprisesadjusting transmit power levels in proportion to local CR base sitenoise floor levels per channel.
 7. The method of claim 4, wherein thestep of determining the CR base stations transmit power level is basedon a geo-location of each available base station.
 8. The method of claim4, wherein the maximum allowed CR mobile subscriber transmit power levelis determined via geo-location of the CR mobile subscriber.
 9. Themethod of claim 4, wherein the maximum allowed CR mobile subscribertransmit power level is determined via spectral sensing.
 10. The methodof claim 4, wherein base site noise floor levels are broadcast to the CRmobile subscriber and taken into account during the step of rankingalong with the local noise level measured at the mobile.
 11. Acommunication system, comprising: a primary system operating underregulated spectrum; a database containing regulation and geo-locationinformation pertaining to the primary system; a secondary system havinga secondary subscriber and a plurality of secondary base stations, thesecondary system seeking to utilize the regulated spectrum; and thesecondary subscriber selecting one of the plurality of secondary basestation channels for operation within the primary system's spectrumbased on secondary base station rankings, the secondary base stationrankings being based on a maximum allowed transmit power level inconjunction with active or passive channel measurements of the secondarysystem.
 12. The communication system of claim 11, wherein the maximumallowed transmit power level is determined based on the geo-locationinformation.
 13. The communication system of claim 11, wherein thesecondary system comprises a public safety network seeking to utilize TVwhite space spectrum.
 14. The communication system of claim 11, whereinthe secondary base station rankings are periodically updated in responseto mobility of the secondary subscriber.
 15. The communication system ofclaim 14, wherein the database containing regulation and geo-locationinformation pertaining to the primary system further includes locationinformation pertaining to the mobility of the secondary subscribercapable of providing updated regulatory and spectrum information basedon change in location of the secondary subscriber.
 16. A method foroptimizing spectrum sharing amongst cognitive radio systems andincumbent systems, comprising: scanning the spectrum for CR basestations by a CR subscriber; detecting the presence of a plurality of CRbase stations; ranking the plurality of CR base stations by the CRsubscriber based on CR base station transmit power, required transmitpower of the CR subscriber to reach each CR base station and estimatedlink budgets, wherein estimated link budgets are based on path lossbetween the CR subscriber and each CR base station of the plurality ofbase stations and local noise levels; selecting a CR base station fromthe ranked CR base station detecting mobility of the CR subscriber andre-ranking the plurality of CR base stations based on the mobility ofthe CR subscriber; and maintaining CR subscriber communication withinthe incumbent spectrum by switching base stations when the detectedmobility meets predefined criteria.
 17. The method of claim 16, whereinthe step of detecting mobility of the CR mobile subscriber is based onat least one of: power of the CR subscriber dropping below that neededto reach the CR base station, a drop in signal quality on the currentoperating channel, and receiving a change in location information from ageo-location database.
 18. The method of claim 16, wherein the CR basestation transmit power is stored in a database.
 19. A method foroptimizing spectrum sharing amongst cognitive radio (CR) systems andincumbent systems, comprising: ranking a plurality of CR base stationsbased on base station transmit power, calculated required CR transmitpower; selecting a highest ranked CR base station for operation withinthe incumbent's spectrum; adapting the ranking based on mobility of theCR subscriber; and the CR system communicating within the incumbent'sspectrum using the adaptively ranked CR base station.
 20. The method ofclaim 19, wherein the ranking is further based on signal to interferenceplus noise ratios (SINR) and local noise levels at the CR base stationand at the CR subscriber.